Temporal variability in offshore Fe fluxes in the Peruvian oxygen minimum zone across an El Niño termination

Abstract. Offshore fluxes of Fe in the Peruvian oxygen minimum zone are sensitive to seawater redox state, and potentially modulated by the El Niño-Southern Oscillation. However, observational data on what controls such temporal variation in offshore Fe fluxes remain scarce. Here, we report seawater concentrations of dissolved Fe (dFe), particulate Fe (pFe), and their isotopic compositions (δ⁵⁶dFe and δ⁵⁶pFe) from two transects at 12° S and 14° S across the Peruvian shelf. Our data show clear shelf-to-slope dFe and pFe plumes with negative δ⁵⁶Fe signatures. Maximum Fe concentrations at 12° S decreased from 13 to 4 nmol kg⁻¹ (dFe) and 122 to 13 nmol kg⁻¹ (pFe) over a month across an El Niño event termination. We link these variations to an intensifying Peru-Chile Undercurrent which enhanced oxygen supply and thereby weakened sedimentary reductive Fe effluxes. Off shelf, elevated δ⁵⁶dFe and low δ⁵⁶pFe in anoxic waters are attributed to organic ligand binding and authigenic Fe formation.